In many conventional manufacturing methods for membrane-type LNG tanks installed in ships or other permanent support structures, tank sections must be separately manufactured at an off-site location and transported for separate installation in a ship's cargo hold or other permanent support structure. Often, the separate sections must be transported with fixtures or assembling devices. After transportation, each section is welded to other partial tank sections in sequence until the entire tank has been assembled in the permanent support structure.
During installation of these tanks, temporary staging and support bracing is required and access to the space between the outside of the tank and the ship's hull must be provided while the sections are being welded together and tank insulation is applied. In addition, internal pumps, piping and tank monitoring systems must be installed before final tank closure and testing. Furthermore, allowance must be made for thermal contraction and expansion of the tank with respect to the ship's hull or permanent support structure.
Such piece by piece erection and installation of a membrane-type LNG tank structure within a ship's hull results in a complicated ship design and an extended shipbuilding schedule. Moreover, when there is a close fit between the LNG tank and the ship's inner hull, access to certain parts of the tank may be restricted and the number of personnel who may simultaneously access the tank to complete the installation may be limited. Furthermore, an attempt to both build the ship and construct the LNG tank within the ship complicates construction and restricts access to necessary building resources such as cranes, welding and ventilation.
One previous attempt at constructing an entire membrane-type LNG tank outside a vessel or support structure includes attaching a temporary support structure to the sides of the tank and attaching a pipe tower between the top and bottom of the tank during manufacture and assembly of the tank. The pipe tower must be placed in the geometric center of the tank and, in order to sufficiently support the tank bottom during movement to the ship, wire ropes or other support devices must be installed from the tower to a uniform pattern of pads welded to the inside of the tank bottom. The tank may then be moved to an installation site, using the temporary support carriage, the pipe tower and the wire ropes to support the tank during the move. The tank may then be installed in a ship's hull or other permanent support structure and the temporary support carriage and wire ropes may be removed.
However, this previous method has given rise to many problems which have not been solved satisfactorily. For example, due to the flexibility of the unstiffened membrane-type bottom, insulation may not be applied to the bottom of the tank until it has been installed in the ship's hull because the insulation lacks the flexibility to move with the flexible bottom during movement and installation of the tank. Therefore, insulation may only be applied to the sides and top of the tank prior to installation of the tank inside the ship's hull. The insulation to be used for insulating the bottom of the tank must be installed to the top surface of a ship's double bottom prior to installation of the tank and, upon installation of the completed tank within the ship's cargo hold, the lower radiused portion of the tank may then be attached to the insulation using a flexible make-up piece.
Additionally, due to the extreme flexibility of the unstiffened tank bottom and the danger of excessive deformation of the bottom plate during lifting and handling, the pipe tower must be located at the geometric center of the tank in order to obtain adequate support of the bottom during lifting and movement of the tank. However, locating the pipe tower at the center of the tank may complicate stripping of LNG from the tank prior to tank inspection or maintenance. Furthermore, upon installation within the ship's cargo hold, the wire ropes attached to the pipe tower and the tank bottom during manufacture and installation must be removed but the welded pads must remain inside the tank and these pads have the potential to begin cracks in the bottom of the tank.
Therefore, there is a need for a method and apparatus for facilitating off-hull manufacturing of a complete semi-membrane LNG tank and installation of the tank in a permanent supporting structure, thereby simplifying the process of manufacturing and installing the tank. Particularly, there is a need for a method and apparatus for manufacturing the tank which allows for attachment of insulation to the tank bottom prior to movement of the tank to and installation in a permanent support structure such as a ship's hull.